FIG. 1 illustrates an example of a general magnetic resonance imaging (MRI) system, which is disclosed in U.S. Pat. No. 7,002,347. For convenience of explanation, the terms and reference numerals used in U.S. Pat. No. 7,002,347 are modified herein. The MRI system includes a magnet 1 generating a base magnetic field, a transmission radiofrequency (RF) coil 2 generating a magnetic field that renders hydrogen nuclei or the like of a patient 4 to enter an excited state, a reception RF coil 3 receiving an RF signal generated when the excited hydrogen nuclei or the like of the patient 4 returns to a ground state, and a table 5 for mounting the patient 4 thereon. The base magnetic field is generally referred to as a B0 field, and the magnetic field generated by the transmission RF coil 2 is generally referred to as a B1 field. The direction of the B0 field is a z-axis direction, and the direction of the B1 field is an x-axis or y-axis direction perpendicular to the B0 field. The transmission RF coil 2 not only generates the magnetic field but also receives the RF signal. The reception RF coil 3 may also generate the B1 field as necessary, as well as receiving the RF signal. Accordingly, unless specified otherwise below, the term “RF coil” is used as having both meanings of a transmitting RF coil and a receiving RF coil.
When a parallel image is particularly obtained by using an MRI system, an RF signal emitted from a patient of whom the parallel image is to be obtained is received via a plurality of RF coils. Since the plurality of RF coils are arranged adjacent to one another, mutual inductance coupling occurs between adjacent RF coils. Due to the mutual inductance coupling, a signal to noise ratio (SNR) of an MRI image degrades. When the RF coils are arranged with a predetermined interval between them in order to reduce mutual inductance coupling, reception of the MRI signal fails due to the interval between the RF coils.
Accordingly, it is important to prevent mutual inductance coupling between adjacent RF coils in order to improve the quality of an MRI image. Various decoupling methods have been developed to prevent mutual inductance coupling.
Prior art related to decoupling methods includes a plurality of patents, such as JP 2000-225106, U.S. Pat. No. 6,150,816, KR 0368890, U.S. Pat. No. 6,927,575, and 6,879,159.
FIG. 2 illustrates an example of a decoupling method using a preamplifier.
The example of FIG. 2 is described in JP 2000-225106, but the terms and reference numerals used in JP 2000-225106 are modified herein for convenience of explanation. An RF coil 6 and a preamplifier 10 are connected to each other via a circuit including an inductor 9. The RF coil 6 includes capacitors 7 and electrical conductors 8. The decoupling method illustrated in FIG. 2 uses a principle that the magnitude of a current flowing in an RF coil is proportional to the amount of mutual inductance coupling occurring between adjacent RF coils. In other words, the magnitude of a current flowing in the RF coil 6 is greatly reduced by greatly increasing an input impedance of the RF coil 6 by using the preamplifier 10 and the inductor 9, and thus mutual inductance coupling occurring between adjacent RF coils may be prevented.
FIG. 3 illustrates an example of a decoupling method in which a decoupling circuit is disposed between adjacent RF coils. The example of FIG. 3 is described in KR 0368890, and the terms and reference numerals used in KR 03688906 are modified herein for convenience of explanation. Three RF coils are arranged adjacent to one another, and are referred to as a first RF coil 20, a second RF coil 30, and a third RF coil 40, respectively. The first RF coil 20 includes capacitors 21 and electrical conductors 22. Similar to the first RF coil 20, each of the second RF coil 30 and the third RF coil 40 include capacitors and electrical conductors. A decoupling circuit 50 is disposed between the first RF coil 20 and the second RF coil 30, which are adjacent to each other. Similarly, a decoupling circuit 50 is disposed between the second RF coil 30 and the third RF coil 40, which are adjacent to each other. By disposing a decoupling circuit between adjacent RF coils as described above, mutual inductance coupling between the adjacent RF coils may also be prevented. The decoupling circuit may include capacitors 51 as shown in FIG. 3.
FIG. 4 illustrates an example of a structural decoupling method of a coil using overlapping between adjacent RF coils. The example of FIG. 4 is described in U.S. Pat. No. 6,879,159, and the terms and reference numerals used in U.S. Pat. No. 6,879,159 are modified herein for convenience of explanation. A first RF coil 60, a second RF coil 70, and a third RF coil 80 are disposed. The first RF coil 60 includes capacitors 61 and electrical conductors 62. Similar to the first RF coil 60, each of the second RF coil 70 and the third RF coil 80 include capacitors and electrical conductors. The first, second, and third RF coils 60, 70, and 80 are arranged such that adjacent RF coils overlap each other to form overlapping portions 90 and 91. The overlapping portions 90 and 91 may prevent mutual inductance coupling from occurring between the adjacent RF coils. In general, when RF coils are square shaped, it is efficient for the area of each of the overlapping portions 90 and 91 with respect to the overall area of adjacent RF coils to be about 14%. When RF coils are circular, it is efficient for the area of each of the overlapping portions 90 and 91 with respect to the overall area of adjacent RF coils to be about 22%.
FIG. 5 illustrates a decoupling method in an RF coil device that resonates at a plurality of frequencies, which is disclosed in U.S. Pat. No. 8,193,811. For convenience of explanation, the terms and reference numerals used in U.S. Pat. No. 8,193,811 are modified herein. In general, an MRI system measures a signal of hydrogen atomic nuclei included in water, fat, or the like. In detail, an RF coil resonates at a Lamor frequency of hydrogen atomic nuclei and obtains measurement information. However, in recent years, it is required to acquire both a signal of hydrogen atomic nuclei and a signal of atomic nuclei other than the hydrogen atomic nuclei. Accordingly, to simultaneously obtain signals of several atomic nuclei, a plurality of RF coils resonating at Lamor frequencies of the several atomic nuclei, respectively, are necessary. FIG. 5 shows RF coil structures 5000 each including two RF coils that simultaneously resonate at Lamor frequencies of two types of atomic nuclei. Each of the RF coil structures 5000 includes a first RF coil 5100 and a second RF coil 5200. In this case, a decoupling circuit 5300 is used to reduce mutual inductance coupling between adjacent RF coil structures.
However, in the decoupling methods described above with reference to FIGS. 3 and 5, an additional decoupling circuit needs to be disposed between adjacent RF coils. In the decoupling method described above with reference to FIG. 4, it is difficult to control a magnetic field generated in an overlap section due to the structural decoupling method using overlapping, and thus, acquiring a pure magnetic field derived from each coil is difficult. Moreover, an RF coil requires tuning in order to resonate at a specific frequency, but the decoupling method using the decoupling circuit or using overlapping affects a resonant frequency of an RF coil and thus, makes it difficult to achieve tuning.